The present invention relates to signal processing and modulation systems and, more particularly, to an improved signal processing system intended for maximizing the efficiency of transfer signals of various frequencies for enhancing signal intelligibility and clarity while avoiding loss of dynamics. The invention has applicability for processing of audio and higher frequency signals transmitted by various transmission modes, such as by radio, TV, and by line systems.
With regard to the transmission of intelligence-carrying signals, including modulating electromagnetic radiation in accordance with a modulating signal (such as audio modulation of a radio frequency signal), there has always been a problem of loss of attenuation of intelligence which is attempted to be transferred within a given frequency band where the frequency band over which intelligence must be transferred is narrower than the range of frequencies which normally convey such intelligence. Thus, when the band width or deviation of a radio frequency signal is narrow, the full spectrum of speech and other audio signals has been heretofore limited. It has, therefore, been general practice merely to transfer only a portion of those frequencies of the audio spectrum with consequent loss of intelligibility, voice character, dynamics, clarity and fidelity in general.
In the line and radio frequency transmission and reception of modulated signals a major problem has always been to obtain a high level of dynamic amplitude while retaining a usable portion of the full spectrum of speech frequencies, while keeping the band width of the transmitted radio signals as narrow as possible.
Relative to transmission of voice signals by modulation of radio frequency signals, attributes of human speech of concern are dynamic amplitude and harmonic relationship. The latter is extremely important in identification intelligibility. Dynamic amplitude can be defined as the varying level of audio received by a modulation stage in any mode of modulation. The human voice is made up by a complex structure of harmonics, the main bands of harmonics falling within a 3 kHz band width. A speech band pass frequency commonly selected is 300 Hz to 3,000 Hz, and all other harmonics are generally suppressed. However, these out-of-bands harmonics define voice character and, thus, intelligibility. But the suppressed harmonics fall in such a wide spectrum that if the entire speech harmonic make-up were to be transmitted, a transmission band width of some 15 kHz would be required. With modern narrow band voice transmission systems, this would become impossible.
There are many modes of radio and line transmission using audio or other signal modulation where these matters are of great concern. Principle forms of modulation presently in use are AM, SSB and FM.